Conventionally, in order to confirm that the required water quality is maintained in ultrapure water production facilities and the like, the number of fine particles in ultrapure water is measured, for example, by an online method applying laser scattering and sound waves, direct microscopy (for example, see Non-Patent Documents 1, 2) or the like.
As a method for measuring the number of fine particles in ultrapure water using the direct microscopy, a method of causing a sampling pipe to branch off from a pipe through which outlet water of an ultrapure water production system flows, filtering part of the ultrapure water through a filtration membrane for measuring the number of fine particles, trapping the fine particles in the ultrapure water on a membrane surface, subjecting this membrane surface to imaging and image processing by a scanning electron microscope or the like, and counting the number of fine particles is proposed (for example, see Patent Document 1). However, when the number of fine particles is counted, it is practically difficult to directly observe the entire membrane surface by the scanning electron microscope or the like since the filtration membrane has a diameter of about 25 mm. Therefore, usually about 0.001% to about 0.1% of an effective filtration area is actually observed by moving a field of view, and the number of fine particles (number of trapped particles) in the ultrapure water trapped by filtration in the entire effective membrane area is found by calculation.
In recent years, further improvement in water quality is required, and a high requirement is also imposed on the number of fine particles in the ultrapure water being one of water quality control items of the ultrapure water. In some cases, the level of the required water quality is, for example, as high as one or less fine particles with a particle diameter of 0.05 μm or more or 10 or less fine particles with a particle diameter of 0.03 μm or more per 1 ml of ultrapure water.
Along with the above requirement of higher water quality of the ultrapure water, the conventional methods for measuring the number of fine particles have the following problems. For example, when a flat membrane is used as the filtration membrane, blank particles (contamination fine particles) which do not result from the ultrapure water as a measuring object inevitably tend to adhere to the surface of the flat membrane in a membrane manufacturing process, a handling process, and the like, since the membrane surface is exposed. As a result, even in the case of a new flat membrane, for example, 105 particles/cm2 to 106 particles/cm2 of blank particles (contamination fine particles) when the measuring object is a particle with a particle diameter of 0.05 μm or more, or 105 particles/cm2 to 107 particles/cm2 of blank particles (contamination fine particles) when the measuring object is a particle with a particle diameter of 0.03 μm or more adhere to the flat membrane in a stage before it is used.
Therefore, to ensure the analytical precision and lower the analytical lower limit, such a volume of water that the number of trapped particles become the same or more than the number of blank particles (contamination fine particles) needs to be passed. For example, when particles with a particle diameter of 0.05 μm or more are measured at a level of 1 particle/ml, 106 ml=1 m3 of filtration volume is needed. Further, when particles with a particle diameter of 0.03 μm or more are measured at a level of 10 particles/ml, 106 ml=1 m3 of filtration volume is needed. As just described, a large filtration volume is needed, and the filtration time tends to be longer.
Moreover, when particles with such a minute particle diameter are measured using the conventional methods, it is necessary to use a filtration membrane with a smaller pore diameter, but the smaller the pore diameter of the filtration filter through which the ultrapure water is passed, the slower the filtration velocity tends to be. For example, the filtration velocity of the flat membrane filter (MF membrane) with a pore diameter of 0.1 μm is 4.0 ml/min (25° C., 0.75 kgf/cm2), and in contrast, in the case of the MF membrane being the flat membrane with a pore diameter of 0.03 μm, the filtration velocity is 0.1 ml/min (25° C., 0.75 kgf/cm2) and decreases greatly.
Hence, for example, a filtration device which shortens the filtration time by using centrifugal force as a pressurizing means is commercially available. However, in such a device, high rotation centrifugal force of a standard rotational frequency of 12,000 rpm is generated, and thereby the device itself becomes expensive and complicated. Further, since the number of blank particles (number of contamination fine particles) adhering to the membrane surface varies, it is necessary to measure a plurality of the numbers of blank particles (numbers of contamination fine particles) of filtration membranes in the same production lot as the filtration membrane used for sampling and to calculate the mean value and standard deviation of the numbers of blank particles (numbers of contamination fine particles) of the filtration membranes.
Non-Patent Document 1: JISK0554-1990
Non-Patent Document 2: “12 years of UCS—Development of Semiconductor Industry and Fruits of 12 years of UCS” edited by Semiconductor Basic Technology Research Association, 12-year Committee, Realize Inc., Sep. 30, 2000, 1. 190-1. 198 pages
Patent Document 1: JP-A 59-83036